Yes this is a concern for the classic motorist, especially as typically the classic car will lay up in a garage for the winter months, only to then find that the petrol has separated and we have a tank with a load of water sitting in the bottom of it.
Another problem for the classic car and bike enthusiast is that the ethanol will attack certain materials commonly found on older vehicles.
Cork gaskets.
Some alloys - that have been used in older carburettors.
Fibreglass - a problem for some off road motorcycles.
There is also a concern that it can increase bore wear.
Ok som I'm not too concerned about bore wear on my Standard 10 as the engine is a bit rattly anyway, But if had an E type, or Healey 3000, I would not be happy at the thought of having to rebuild my engine prematurely.

Nick